The term "extensions" is also weird. It seems to be used for two things: One, "libraries" (any standard types and functions they wanted to include but don't inherit directly from the CLR's library). Two, a "Go-shaped" (*sigh*) "extension" that is both a "library" (some trivial functions like `len`) and also changes the language by adding channels and some new syntax! Wow, syntax can be imported with an import statement! But only this specific syntax, with this specific import!
And this is "opt-in", and an extension. But... it's right there. It ships with the language. It is in the language. That's not what an extension is to normal humans. And opt-in, sure. I guess I wouldn't have to use it. Just like every other language feature I don't feel like using?